Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface, showing the new design.

The dotted lines showing portions of the graphical user interface illustrate environmental subject matter and form no part of the claimed design.

The dash-dot-dash line showing of a display screen or portion thereof defines the bounds of the claimed design and forms no part thereof. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 